Acetic acid production by carbonylation includes continuously reacting methanol and carbon monoxide in the presence of a catalyst in a reactor. The reaction mixture present in the reactor comprises a transition metal, which may be a Group VIII metal, which may be iridium and/or rhodium or nickel, and may further include one or more solvents, water, various stabilizers, co-catalysts, promoters, and the like. Reaction mixtures known in the art may comprise acetic acid, methyl acetate, methyl iodide, hydrogen iodide, a hydrogen iodide promoter, and the like.
A complex network of inter-dependent equilibria involving liquid acetic acid reaction components exists within the reaction mixture present in the reactor, which include those directed to the formation of acetic acid, as well as those directed to the formation of various impurities which are also produced in the reactor. Impurities which may be present in acetic acid include permanganate reducing compounds (PRCs) such as acetaldehyde.
Hydrogen iodide, HI, whether present in molecular form as hydrogen iodide or dissociated in a solvent as hydriodic acid, is present in the reaction mixture according to various production schemes. However, HI is highly corrosive to metals and presents various challenges in acetic acid production.
Attempts in the art are directed to minimizing HI outside of the reactor, with the ultimate goal of eliminating it outside of the reactor. Examples include processes directed to injecting methanol into a distillation column with the purported intention of reacting the methanol with the HI present therein to form methyl iodide and water. (cf. JP2000-72712, Japanese application 10-244590, filed Aug. 31, 1998, the entire contents and disclosure of which is hereby incorporated by reference). However, processes directed to minimizing HI to reduce or eliminate corrosion issues failed to realize benefits associated with particular concentrations of HI outside the reactor. There is a need to control HI concentrations within particular ranges in various aspects of an acetic acid production process.